The invention relates generally to an instrument for measuring moisture content.
More particularly, the invention relates to an instrument for measuring the moisture content of dielectric substances, especially dielectric solids.
A known instrument for measuring the moisture content of dielectric objects has a housing which accommodates a measuring circuit designed to detect small changes in capacitance. A measuring capacitor capable of generating a diverging voltage field extends from the housing.
The measuring capacitor in a conventional instrument of this type consists of a plurality of parallel or concentric electrodes. The electrode arrangement includes both active and grounded electrodes and is designed in such a manner that the active electrodes alternate with or are located adjacent to the grounded electrodes. In other words, each active electrode is located between two grounded electrodes.
In order to determine the moisture content of an object, the electrodes are placed against the object. Conventional instruments of the type under consideration are designed on the assumption that the spacing between the electrodes must be smaller than the dimensions of the object. It is believed that virtually the entire voltage field of the measuring capacitor then penetrates into the object.
In these moisture measuring instruments, the depth of penetration of the voltage field into the object is highly unsatisfactory. Thus, the field strength decreases significantly within a few millimeters. This results in substantial errors when, as is the case for many solids, the moisture distribution in an object is very non-uniform. For example, structural components consisting of synthetic resins may exhibit a non-uniform moisture distribution when used under extreme conditions since they may develop hairline cracks or pores which can trap water. If a moisture determination on such a structural component is made at an inner surface thereof, any moisture which may, for instance, have accumulated in hairline cracks at the outer surface cannot be detected. The same applies, for example, to moisture determinations performed on wood where a non-uniform moisture distribution along the thickness develops during drying.